International Journal of Agriculture and Biological Sciences- ISSN (2522-6584) July & Aug 2020 August 31, 2020 Page 102 Study of the evolution of the ethyl ester content of extra virgin olive oil after one year of storage and its impact on the quality of the oil Author’s Detials: Assia Rabeh 1 , Nadia Maata 2 , Ahmed Adlouni 1 1 Unit of metabolic and Immune pathology, Biology and Health Laboratory URAC 34 Faculty of Sciences Ben Msik Hassan II University of Casablanca, B.P 7955, Sidi Othman Casablanca 20100 Morocco. 2 Laboratory of Analysis and Chemical Research of Casablanca, Morocco LOARC 25, Rue Rahal Nichakra - Ex Tours Casablanca 20100 Morocco. Corresponding author: Assia RABEH Mobile: (212) 0619634664 Received Date: 13-Aug-2020 Accepted Date: 28-Aug-2020 Published Date: 31-Aug-2020 ______________________________________________________________________________________________ Abstract This study was carried out on 86 olive oil samples from the 2016 olive growing season, in three Moroccan regions; eastern region, central region and northern region, by analyzing the free acidity by titrimetric test (ISO 660) and the ethyl ester content by gas chromatography method 3 grams (COI / T.20 / DOC N ° 31 November). The samples were stored under special conditions and the study took place over two periods 6 and 12 months of storage. Taking into account the acidity analysis, the results obtained showed no change after 6 months of storage. We conclude that 17.44% of the samples degrade passing from the extra virgin category to a virgin category. For the ethyl ester content, 4.6% of the samples were degraded after 6 months of storage and 29.06% were degraded after one year of storage. In addition, we observed that the growing region of olive trees has an impact on the free acidity and the ethyl ester content of olive oil. For this criterion, we observed that in the central region of Morocco, the olive oil samples are the least degraded compared to other regions. Thus, concerning the content of ethyl esters, 13% of the samples come from the central region, 37% from the eastern region and 39% from the northern region. On the other hand, for the analysis of the acidity, 8% of the samples come from the center, 30% of the samples come from the oriental and 17.39% of the samples come from the north. So we can conclude that for the conservation of extra virgin olive oil, the olive trees of the central region of Morocco can benefit from a good storage while retaining their high nutritional quality proven by the analysis of acidity and the content of ethyl esters. Key words: quality, free acidity, ethyl ester, conservation, region. ______________________________________________________________________________________________ INTRODUCTION: Olive oil is a biological product whose history goes back to antiquity. It was used as medicine and staple food by the whole Mediterranean population (Bejaoui et al., 2018). Outside the Mediterranean, countries where the olive oil consumption has always been rare, especially the United States, have begun in recent years to increase the demand for olive oil, representing 9% of world consumption (Costa et al., 2017). In addition, nutritionists are increasingly inclined to propose the Mediterranean diet as a healthy way against the dietary habits of modern life, rich in protein saturated fat, while olive oil offers a variety of unsaturated fatty acids including oleic acid and polyphenols (Bejaoui et al., 2018; Costa et al., 2017). In Morocco, olive growing is considered as a central socio-economic sector (Ater et al., 2015), since it is the main fruiting species in rapid extension, which plays a crucial role in agricultural development and especially in the framework of the Green Morocco Plan, where an olive orchard extension areas is planned to reach 1.2 million hectares and olive production of 2.5 million tonnes by 2020 (Ater et al., 2015). These orchards are scattered over different Moroccan regions where we found; the central region (Fez-Meknes), northern region (Tangier-Tétouan- Al Hoceima), Orientale region, and the southern provinces, whose predominant variety is Moroccan picholine that adapts well to the soil and climatic conditions of the whole country and has typical characteristics of double aptitude variety. The produced olives are used for all confectionery types: broken green olives with olives ripe black, having yield oil average (Meftah et al., 2014). Due to the well-known features and benefits of health (Fortini et al., 2015), extra virgin olive oil has become a target for illegal